What subject matter are you shooting that needs f/22? That seems to be about the only aperture that dust would be s problem. The onset of diffraction starts well below that on a D600.
I am amazed how many people are believing dust that is always present is a reason to return a perfectly good camera. I doubt anyone just 5 years ago would have done so. The worst that could happen is oil is leaking from a motor seal and in which case Nikon will replace the motor assembly but that is not what people are complaining about, it appears to be dust. Giving it a good blowing out before cleaning the AA filter is probably the solution.
The claims of oil on the sensor does not seem to have much evidence to support it but you know how internet rumors are. Get some cleaning supplies, you are going to need it for general maintenance for any brand, and learn how to clean the camera. It is easy and fast. But blow the whole camera cavity out before since there is probably some residue from assembly that if left in side the camera will just settle on the AA filter again. If I liked the features and image quality I would not hesitate in getting one now but I already have a camera that I am thrilled with, a D800. The D600 is probably the second best imaging device for pure IQ, low noise and artifact, no banding in the DSLR format camera, across any brands. Stan St Petersburg Russia